The electrolytic production of chlor-alkali the most widespread process in the electrochemical field. This process utilizes sodium chloride which is converted into sodium hydroxide and chlorine by applying electric current.
Also known, even if not so common, is the process based on the use of potassium chloride as starting material, to obtain potassium hydroxide and chlorine as final products. Chlorine and caustic soda may be also produced respectively according to the methods schematically resumed as follows:
electrolysis or catalytic oxidation of hydrochloric acid, available in large amounts as a by-product of the chlorination of organics. Hydrochloric acid may be further obtained by a reaction between sodium chloride and sulphuric acid, with the side-formation of sodium sulphate; PA1 causticization of a sodium carbonate solution with lime, subsequent filtration of the by-produced solid calcium carbonate and concentration of the diluted solution of sodium hydroxide containing various impurities deriving from the lime and from the sodium carbonate solution. PA1 Sodium carbonate is commonly produced by the process developed by Solvay, based on the conversion of sodium chloride brine into sodium bicarbonate, which is scarcely soluble, by means of a chemical reaction with ammonia, which is then recycled, and carbon dioxide. Bicarbonate is then converted into sodium carbonate by roasting. PA1 The raw materials comprise, therefore, sodium chloride, lime and carbon dioxide, both obtained from calcium carbonate, and the ammonia necessary to make up for the unavoidable losses. PA1 A further source of sodium carbonate is represented by trona or nahcolite mineral ores which contain sodium carbonate and bicarbonate and minor percentages of other compounds, such as sodium chloride.
It is evident that the above alternatives are based on complex processes which involve high operation costs. For these reasons, these processes were gradually abandoned in the past and the market become more and more oriented towards the chlor-alkali electrolysis process which is intrinsically simpler and energy-effective due to the development of the technology based on mercury cathode cells progressively evolved to diaphragm cells and now to membrane cells. However, chlor-alkali electrolysis is today experiencing a decline, which is connected to the rigid stoichiometric balance between the produced quantities of sodium hydroxide and chlorine. This rigid link was no problem when the two markets of chlorine polyvinyl chloride or (PVC, chlorinated solvents, bleaching in paper industry, various chemical reactions) and of sodium hydroxide (glass industry, paper industry, various chemical uses) were substantially balanced. Recently, a persistent downtrend in the chlorine market (reduced use of PVC and chlorinated solvents, decreasing use in the paper industry) combined with a robust demand of caustic soda, seemingly bound to increase in the near future, pushed the industry towards alternative routes for producing sodium hydroxide without the concurrent production of chlorine, in some cases even considered an undesirable by-product. This explains the revival of the sodium carbonate causticization process, notwithstanding its complexity and high costs.
In this scenery, the electrochemical industry is ready to propose alternative processes evolving from the existing ones (see C. L. Mantell, Industrial Electrochemistry, McGraw-Hill) and made more competitive by the availability of new materials and of highly selective ion exchange membranes. The most interesting proposal is represented by the electrolysis of solutions of sodium sulfate, either mined or as the by-product of various chemical processes. Electrolysis is carried out in electrolyzers made of elementary cells having two electrolyte compartments separated by cation-exchange membranes or in a more sophisticated design, electrolyzers made of three electrolyte compartment elementary cells containing anion- and cation-exchange membranes. This process, also known as sodium sulfate splitting, generates sodium hydroxide (15-25%), hydrogen, oxygen and, in the simplest design, diluted sodium sulphate containing sulphuric acid, or in the more sophisticated design, diluted sodium sulphate and pure sulphuric acid. While sodium hydroxide is a desirable product, pure sulfuric acid and even more the acid solution of sodium sulfate pose severe problems. fact, if these products cannot be recycled to the other plants in the factory, they must be concentrated, with the relevant high costs, before commercialization in a rather difficult market usually characterized by large availability of 96-98% sulphuric acid produced at low cost in catalytic large-scale plants. The evolution of oxygen at the anodes of the elementary cells of the electrolyzer further involves a high cell voltage, indicatively 3.5 Volts for the simpler design and 4.5-5 Volts for the more sophisticated design, operating in both cases at 3000 Ampere/m.sup.2 of membrane. These high voltages implicate a high energy consumption (2,700-3,700 kWh/ton of caustic soda).
A method to solve the above problems is offered by the process disclosed in U.S. Pat. No. 4,636,289, K. N. Mani et al., assigned to Allied Corporation. According to the teachings of this patent, an aqueous solution of a sodium salt, preferably sodium sulfate, is fed to an electrolyzer equipped with bipolar membranes (water splitter) and the outlet acid stream comprising diluted sodium sulfate and sulfuric acid is neutralized by sodium carbonate, sodium bicarbonate or mixtures thereof. The resulting neutral sodium salt solution is purified and recycled to the water splitter (indirect electrolysis). Even if not specifically said in U.S. Pat. No. 4,636,289, this process permits to obtain caustic soda with limited energy consumptions (1500-2000 kWh/ton of caustic soda). The problem affecting this technology is represented by the weakness of the bipolar membranes which are attacked by oxidizing substances, require low current densities (in the range of 1000 Ampere/m.sup.2), an extremely efficient purification of the sodium salt solution to remove bivalent metals, such as Mg.sup.--, relatively low acid concentrations, with an increase of the operation costs due to the high flow rates of the solutions to be recycled. Further, also under the best operating conditions, the bipolar membranes are characterized by a rather short lifetime, in the range of about 1 year. These drawbacks may be overcome by substituting the water splitter described by Mani et al. with electrolyzers constituted by elementary cells divided in two electrolyte compartments by cation-exchange membranes and provided with oxygen-evolving anodes as previously described. These electrolyzers, as already said, have high energy consumptions but offer several important advantages. In fact, the cation-exchange membranes have a very satisfactory lifetime, over 2 years, typically 3 years, and are capable of operating under high current densities, around 3000 Ampere/m.sup.2. As regards the content of bivalent metal ions, such as Mg.sup.--, the required tolerance limits are not so strict as for water splitters equipped with bipolar membranes. However, certain impurities, such as organic substances and chlorides, must be kept under control as they could cause a premature deactivation of the oxygen-evolving anodes. Further, chlorides are oxidized to chlorine which mixes with oxygen, the main product of the process, in which event oxygen must be subjected to alkaline scrubbing to absorb chlorine, before release to the atmosphere.
A system to decrease the energy consumption electrolyzers is found in the technical literature, for example H. V. Plessen et al. --Chem. Ing. Techn. 61 (1989), N. 12, page 935. According to this teaching, the oxygen-evolving anodes may be substituted with gas diffusion anodes fed with hydrogen. Such gas diffusion anodes comprise a porous sheet containing a catalyst dispersed therein and are suitably made hydrophobic, in order to maintain the liquid immobilized inside the pores, as taught for example in EP 0357077. However, this kind of anode is completely unreliable when its dimensions are increased for example up to one square meter, as required by industrial applications and it is inserted in a high number of cells, as it is the case in commercial electrolyzers. In fact, unavoidable percolations of liquid take place in those areas where defects are present due to manufacturing or mishandling. These percolations prevent hydrogen from reaching the catalytic sites and cause dangerous plugging of the hydrogen circuit. Further, the solution coming into contact with the catalyst inside the pores of the sheet may cause deactivation when certain impurities are present, such as heavy metals frequently found in the solutions to be electrolyzed. Moreover, if the solution in contact with the catalyst contains reducible species which easily react with hydrogen, undesired by-products are formed and the process efficiency is decreased.
These shortcomings of the hydrogen depolarized anodes are overcome by the assembly disclosed in U.S. Pat. No. 3,124,520. According to the teachings of this patent, the hydrogen-depolarized anode assembly comprises a cation-exchange membrane and a porous electrocatalytic sheet in face-to-face contact. The membrane protects the sheet against percolations of the electrolyte and prevents contact between the catalyst particles of the sheet and poisoning impurities or reducible substances contained in the electrolyte. The teaching of U.S. Pat. No. 3,124,520 applied to sodium sulfate electrolysis is found in U.S. Pat. No. 4,561,945 where also construction details are illustrated. In particular, according to U.S. Pat. No. 4,561,945, the electrocatalytic sheet is obtained by sinterization of a mixture of catalyst particles and polymer particles and by bonding of the sinterized electrocatalytic sheet to the surface of the membrane by application of heat and pressure. This particular type of construction is made necessary as with the hydrogen depolarized anode assembly of U.S. Pat. No. 4,561,945, the catalyst particles of said electrocatalytic sheet are in contact only with hydrogen gas and with the membrane, no electrolyte being present on this side of the membrane but just on the opposite side. As the conductive path ensured by the electrolyte is not provided, the ionization of hydrogen may take place only in the points of direct contact between the catalyst particles and the membrane. The remaining surface of the catalyst particles not in contact with the membrane results completely inert. As a consequence, in order to obtain a useful current density for industrial applications it is required that a great number of individual particles contact the membrane at a plurality of points. This requirement may be accomplished according to the state of the art teachings only by bonding the membrane and the electrocatalytic sheet. It is soon apparent that said fabrication method is particularly expensive and intrinsically unreliable when applied to electrodes of large unit area, in the range of 1-2 square meters each, to be produced in a large quantity, in the order of some hundreds of pieces for each production lot. Actually, powerful pressing devices are required, working at controlled temperature and there is a remarkably high possibility that the membrane during pressing and heating be punctured or cracked if excessively dehydrated.